Environmental standards for particulate emissions by coal-fired electrical power plants, petroleum refineries, chemical plants, pulp and paper plants, cement plants, and other particulate-emitting facilities are becoming increasingly more demanding. For example, air quality standards in the United States now require power plants to remove more than 99% of the flyash produced by coal combustion before the flue gas can be discharged into the atmosphere. As environmental standards tighten, there is a corresponding need for a more efficient means of particulate removal.
An electrostatic precipitator is a commonly used device for removing undesired particles from the gas streams produced by plants and refineries. As used herein, "undesired particles" refers to any particulate matter such as flyash, that is desired to be removed from a gas stream. In a typical electrostatic precipitator, undesired particle-laden gases pass negatively charged corona electrodes which impart a negative charge to the undesired particles. The charged particles then migrate towards positively charged collection plates and are removed by various techniques, including sonic horn blasts or rapping of the collection plates. Electrostatic precipitators may employ a common stage or separate stages for both the charging and collection of undesired particles.
In utility applications, there are two types of electrostatic precipitators. Cold-side electrostatic precipitators are located on the downstream side of the air preheater or heat exchanger (which transfers heat from the flue gas to the air to be fed into the furnace) and therefore operate at relatively low temperatures (i.e., temperatures of no more than about 200.degree. C.). Hot-side electrostatic precipitators are located on the upstream side of the air preheater and therefore operate at relatively high temperatures (i.e., more than about 250.degree. C.).
Many hot-side electrostatic precipitators suffer from problems related to the resistivity of collected undesired particles. Such problems can cause a deterioration of the particulate collection efficiency of the electrostatic precipitator and excessive power consumption. These problems can be caused by sodium depletion of collected undesired particles on the collection plates, inherently high resistivity of undesired particles, or resistivity problems during low load or at colder temperatures.
Additives, such as sulfur trioxide, ammonia, and various surface conditioning additives (such as sulfuric acid) that are effective under cold-side conditions are generally ineffective under hot-side conditions because of different charge conduction mechanisms. Referring to FIG. 1, under cold-side conditions (which exist at operating temperatures less than the critical temperature) surface conduction of charge is the predominant charge conduction mechanism while under hot-side conditions (which exist typically at operating temperatures more than the critical temperature) volume conduction of charge is the predominant charge conduction mechanism. As used herein, the "critical temperature" is the temperature corresponding to the highest attainable resistivity of an undesired particle (which is commonly located at the top of a bell-shaped curve as shown in FIG. 1).
One conditioning method for controlling high temperature resistivity that has had some success under hot-side conditions has been bulk addition of sodium into the coal feed to the boiler. Typically, from about 0.5 to about 3% by weight sodium is added to the coal feed as a sodium sulfate or soda ash. The sodium is co-fired with the coal in the boiler and is incorporated into the undesired particles as various sodium oxides. However, the bulk addition of sodium to the coal feed can cause serious problems, such as boiler slagging due to high sodium flyash, the consumption of excessive amounts of alkali material and a commensurate increase in operating costs, higher gas temperatures downstream of the boiler that can aggravate duct and electrostatic precipitator structural problems, and an inability to supply the additive on an intermittent or as-needed basis.